1. Field of the Invention
This invention relates to a lens meter which can accurately recognize measuring points for a framed progressive lens or an unidentified progressive lens.
2. Description of the Prior Art
A progressive lens having no boundary of far and near vision region has been conventionally used for correction of presbyopia. A progressive band of this progressive lens is structured in a consecutive manner on a very complicated non-sphere surface. Thus, the far and near vision region cannot be distinguished easily from the appearance, which is possible, for example in a bifocal lens. Therefore, lens manufacturers provide various kinds of marks or hidden-marks showing a far vision measuring point, a near vision measuring point, a geometric center or an optical center. When measuring spherical power in the far vision region by using a lens meter, a mark of the far vision region is adjusted to an axis of measuring light of the lens meter. Conversely, when measuring spherical power in the near vision region, a mark of the near vision region is adjusted to the axis of measuring light of the lens meter.
It is sometimes necessary to measure a framed progressive lens (spectacle lens) of which periphery of an uncut lens is cut out and put in the glass frame. Since the periphery of the framed progressive lens is ground, some of the marks provided on the uncut lens, including the far and near vision marks, are disappeared. In addition, since the framed progressive lens is not shaped in a symmetrical manner, it is difficult to distinguish the far and near vision marks from their appearances.
Therefore, each manufacturer provides a lens mark sheet so that an optical retailer can measure the far and near vision region of the lens based on the hidden-marks and the lens mark sheet. However, the optical retailer does not always have all of the mark sheets for each manufacturer. Even if they do, it is not easy to keep the mark sheets in good condition. In addition, it is not only difficult to find the hidden-marks, but also time-consuming to place the mark sheet on the framed progressive lens and measure it thereby.
As disclosed in Japanese Patent Laying-Open Publication No. 61-200441, it has been conventionally suggested to measure the far and near vision power by moving a dimension designated by a manufacturer from an optical center of the framed progressive lens. However, this measuring method has a disadvantage that the far and near vision power cannot be measured if a type of the progressive lens is not known in advance.
Throughout the disclosure of the present invention, spherical power designates refracting power of a spherical lens, add power designates relative value of the spherical power of a measuring point with respect to the spherical power in a far vision region in the progressive lens, and cylindrical power designates refracting power of a cylindrical lens. Further, a cylindrical axis indicates a direction when the cylindrical power is zero, prismatic power designates refracting power of a prism, and a progressive band designates an area of the lens having a prescribed width where the cylindrical power is constant while the spherical power varies consecutively.